The present invention relates to a polymer electrolyte fuel cell, and particularly to an improvement in electrodes constituting the polymer electrolyte fuel cell.
In the electrodes of a polymer electrolyte fuel cell, the discharge performance of the cell is determined by the area of the so-called “three-phase interface”, which is formed by pores serving as a supply path of a reactant gas, a hydrogen-ion conductive polymer electrolyte, and a catalyst material serving as an electron conductor. In order to increase the three-phase interface and reduce the amount of noble metal which is the catalyst material, attempts have been made to mix the hydrogen-ion conductive polymer electrolyte with the catalyst material and disperse it. For example, Japanese Laid-Open Patent Publications No. Sho 61-295387 and No. Sho 61-295388 propose a method by which a dispersion of a hydrogen-ion conductive polymer electrolyte is mixed with a catalyst material, the mixture is applied onto a hydrogen-ion conductive polymer electrolyte membrane, the resultant membrane and electrode materials are hot pressed, and the catalyst material is reduced to a metal catalyst.
Such a polymer electrolyte fuel cell has a following problem: due to cross leak of hydrogen gas from an anode side to a cathode side through a solid polymer electrolyte membrane, the hydrogen partial pressure increases on the cathode side, so that the difference between the chemical potential of hydrogen at the anode and that at the cathode is lessened, thereby resulting in lowered electromotive force. Also, it has been reported that the hydrogen gas which has cross-leaked from the anode side burns locally with oxygen gas in the catalyst layer of the cathode, thereby to deteriorate the polymer electrolyte membrane.
In order to suppress the cross leak of hydrogen gas, Japanese Laid-Open Patent Publication No. Hei 6-103992, for example, discloses a technique in which a catalyst metal is carried in a polymer electrolyte membrane in order to react hydrogen gas and oxygen gas. Further, Japanese Laid-Open Patent Publication No. Hei 8-88008 discloses a technique in which the amount of a fluorocarbon sulfonic acid polymer resin is decreased in the catalyst layers of both anode and cathode in order to react hydrogen gas and oxygen gas that have cross-leaked.
However, the deterioration of the hydrogen-ion conductive polymer electrolyte due to the hydrogen gas and oxygen gas occurs not only in the polymer electrolyte membrane but also in the catalyst layers. Particularly, it is impossible to introduce, into the catalyst layers, a large amount of the hydrogen-ion conductive polymer electrolyte because of the necessity for ensuring diffusion of the reactant gas, so that deterioration of only a small amount of the hydrogen-ion conductive polymer electrolyte constitutes a significant loss in the catalyst layers. Therefore, in the polymer electrolyte fuel cell, the deterioration of the hydrogen-ion conductive polymer electrolyte due to local combustion of hydrogen has a larger impact on the catalyst layers than on the hydrogen-ion conductive polymer electrolyte membrane. The deterioration of the hydrogen-ion conductive polymer electrolyte is remarkable in the catalyst layer of the cathode.
Because of the deterioration of the hydrogen-ion conductive polymer electrolyte, the power generating efficiency of the fuel cell lowers with passage of its operating time. However, when the humidity of the oxidant gas supplied is high and the catalyst layer is therefore supplied with sufficient water, the loss of the hydrogen-ion conductive polymer electrolyte can be suppressed, since the hydrogen-ion conductive polymer electrolyte is swollen with water and heat generated by combustion is therefore scattered and lost. When the humidity of the oxidant gas supplied is low, on the other hand, severe decomposition of the hydrogen-ion conductive polymer electrolyte occurs to cause a considerable reduction in the power generating efficiency, since the hydrogen-ion conductive polymer electrolyte in the catalyst layer is dry and heat generated by combustion concentrates.